The present invention relates to a substrate for use with electronic devices and particularly one formed at low temperature having a low dielectric constant.
Conventionally, alumina has been used as the material to form a substrate for electronic apparatus. As semiconductor devices have increased in integration (LSI's, VLSI's) and as the requirements for high response speed and large scale have affected the production of electronic devices, interest has been shown in substrates. The substrates formed at low temperature include glass composite substrates, crystalline glass substrates and non-glass substrates. These materials are more advantageous in that they can be formed together with low resistance conductors having lower or dielectric constants than the prior art alumina substrates.
Recent developments in high speed computer design, however, have made further demands in lowering the dielectric constant necessary for the substrate because the frequency used in the electric elements mounted thereon have become higher and higher, up to the GHz range.
Such applications have created interest in lowering the dielectric constant of the substrate. For example, a substrate has been proposed that is made from a crystalline compound oxide and silicon dioxide as main components, having pores in the substrate not larger than three microns (.mu.m) in size with a porosity in the range of from 10 to 30 volume percent to lower the relative dielectric constant to a value of not higher than 0.5 at 1 megahertz. See Japanese Patent Unexamined Publication No. 61-278194. In such a substrate, however, the practical lower limit of the dielectric constant is 4.0 because of constraints imposed by the needed mechanical strength of such a substrate. There has also been proposed a complex layered substrate having a front and rear surface layer made of a material having a mechanical strength of not less than 15 Kg/mm.sup.2 An intermediate layer, disposed between the two surface layers, is formed of a material having a dielectric constant of not more than 5 (Japanese Utility Model Unexamined Publication No. 61-134081). In forming this layered substrate, however, the materials in the outer layers are different than the center layer so that the bonding between the different materials is potentially difficult with a risk of deterioration of the bond between the layers.
Therefore, one object of the present invention is to provide a substrate that can be formed at low temperature having sufficiently high mechanical strength with a coefficient of thermal expansion near to that of a silicon chip in which the dielectric constant can be lowered. As a result, signal delay time of electric components associated with the substrate can be shortened to cope with the trend in electronic devices requiring higher and higher response speeds. Another object of tile present invention is to provide a method of manufacturing a substrate than can be formed at low temperature in mass production techniques that are economically feasible.